In the field of internal combustion engines, it is often desirable to measure the level of various engine liquid levels, such as engine oil, fuel, coolant, etc. In the case of engine oil, for example, engines are nearly always provided with an oil dipstick which is extended down a tube and into the engine oil pan. By withdrawing the dipstick, the amount of oil in the oil pan may be roughly determined by visually inspecting the distance from the distal end of the dipstick to the uppermost reach of the oil clinging thereto.
This method for measuring oil exhibits several drawbacks. First, the visual measurement is an extremely vague measurement in terms of quantity of oil. In some instances, for example, it is desired to measure the amount of oil consumed by an engine versus time, in a long-term test in which the engine is operated continuously over an extended period. In such instances, a dipstick does not provide satisfactory measurement resolution or repeatability. Also, dipsticks are inconvenient from a measurement time standpoint, such as for use in fleet vehicles where it is desired to measure the oil level on a daily basis.
Attempts have therefore been made in the prior art to provide for automated electronic sensing of fluid levels such as engine oil levels. Such measurements can be problematic due to the harsh environment in which the measurement must be made. For example, the use of capacitive sensors in the oil pan are generally unreliable due to the high soot environment of the engine oil. Furthermore, oil pans experience high vibration levels when the engine is operating, making it difficult to maintain pressure transducer based measurement systems in the oil pan. Also, oil pan durability can be compromised when mounting sensors therein.
Another problem encountered when trying to measure oil level in the oil pan by measuring the pressure exerted by the oil on a pressure sensor is the inaccuracy of such measurements caused by varying head pressure above the oil (caused by blowby gases and the dynamics of the moving engine parts). Varying head pressure can also be a problem in other closed vessels, such as fuel tanks and coolant tanks.
There is therefore a need for an accurate liquid level measurement system that may be read automatically, that avoids reliability problems associated with a harsh measurement environment, and that produces an accurate level measurement, even with varying head pressures. The present invention is directed toward meeting this need.